Ferromagnetic thin film magnetic recording medium having an intermediate layer containing ultrafine particles and a polar group containing binder resin having a loss tangent peak temperature in a specified range

ABSTRACT

A ferromagnetic thin film type magnetic recording medium is disclosed, comprising a nonmagnetic intermediate layer formed between a nonmagnetic support and a magnetic layer, the intermediate layer containing a binder resin and ultrafine particles, wherein the ultrafine particles have a diameter of from 5 to 250 nm and (a) the binder resin has as a polar group at least one group selected from the group consisting of --SO 3  M, --OH, --OSO 3  M, --COOM, ##STR1## --NR 2  and --⊕NR 3  X⊖ wherein M is H, Li, Na, K or a hydrocarbon group, R is H, or an alkyl group, and X is a halogen atom, or (b) the peak temperature of the loss tangent (tan) of the binder resin is from 10 to 90° C. 
     The magnetic recording medium has an excellent running durability, still life and frictional property.

FIELD OF THE INVENTION

The present invention relates to a magnetic recording medium, and moreparticularly it relates to a ferromagnetic thin film type magneticrecording medium for higher density recording, which has an excellentrunning durability, still durability (still life), and frictionalproperty.

BACKGROUND OF THE INVENTION

In order to increase the recording density of a high density magneticrecording medium, there has been used a nonmagnetic support film havinga very smooth surface. When the surface of the nonmagnetic support filmis smooth, the frictional resistance thereof against a conveying rollerbecomes large, thereby frequently causing winding or wrinkles. Further,the frictional resistance between the nonmagnetic support films areincreased so that a winding roll is liable to be warped.

Recently, using a deposition or sputtering, etc., there has beendeveloped a magnetic recording medium comprising a ferromagnetic thinfilm made of Co-Ni, Co-Cr, or Fe-N, etc. However, since theferromagnetic thin film is much more smooth and thin than a magneticlayer formed by coating which comprises mainly ferromagnetic particlesand a binder resin, the above-mentioned problems are further increasing.

In order to solve such problems, various attempts have been made, one ofwhich is to provide an intermediate layer of a coating type between amagnetic layer and a support. Depending on the situation, suchintermediate layer can be called an under coat.

For example, in JP-A-56--116115 (the term "JP-A" as used herein means an"unexamined published Japanese patent application"), JP-A-53--128683,JP-A-54--94574, JP-A-56--10455, and JP-A-56--16937, the surface of anonmagnetic support is finely and uniformly roughened so that it canwell touch a magnetic head and the running property can be improved.

JP-A-58--68227 discloses a method in which a ferromagnetic thin film isformed on a plastic film, the surface of which has a granular film towhich fine particles having a size of 0.01 to 0.2 μm or is formed withworm-like nodules are added.

JP-A-59-48825 proposes that colloidal silica and binders are used toform an undercoating layer, thereby forming 1000 pieces/mm² or more ofgranular protrusions of 30 to 500 Å high.

However, in these prior arts, it is difficult to form on a support anundercoating layer in which fine particles are uniformly dispersed inbinders. Namely, there is formed on a support an undercoating layer inwhich fine particles are not uniformly dispersed but aggregate therein.

As a result, there are problems that clearance loss between a magnetichead and a magnetic layer is increased, thus lowering the output; stilllife is made to be shortened; and running durability is made to belowered.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a ferromagnetic thinfilm type magnetic recording medium for higher density recordingcomprising a nonmagnetic intermediate layer in which ultrafine particlesare uniformly dispersed, and the running property as well as durabilityare excellent.

According to the present invention, there is provided a ferromagneticthin film type magnetic recording medium comprising a nonmagneticintermediate layer formed between a nonmagnetic support and a magneticlayer, the intermediate layer containing a binder resin and ultrafineparticles, wherein the ultrafine particles have a diameter of from 5 to250 nm and (a) the binder resin has as a polar group at least one groupselected from the group consisting of --SO₃ M, --OH, --OSO₃ M, --COOM,##STR2## --NR₂ and --⊕NR₃ X⊖wherein M is H, Li, Na, K or a hydrocarbongroup, R is H, or an alkyl group, and X is a halogen atom, or (b) thepeak temperature of the loss tangent (tanδ) of the binder resin is from10 to 90° C., preferably from 15 to 85° C., particularly preferably from18 to 80° C.

BRIEF DESCRIPTION OF THE DRAWING

The present invention can be more fully understood from the followingdetailed description when taken in conjunction with the accompanyingdrawing.

The drawing shows a view showing a winding type vacuum vapor depositiondevice used for forming a magnetic recording medium of the presentinvention, in which each numeral indicates as follows:

1: vacuum chamber;

2: upper room;

3: vapor deposition room;

4, 5: vacuum exhaust opening;

6: non-magnetic support;

7: supply roll;

8: winding roll;

9: cold can (rotating drum);

10: evaporation boat;

11: magnetic material;

12: electrogun;

13: mask;

14, 15: gas inlet;

16: vapor flow; and

17: incident angle.

DETAILED DESCRIPTION OF THE INVENTION

In the magnetic recording medium of the present invention, wherein thenonmagnetic intermediate layer contains a binder resin having at leastone polar group selected from the group consisting of --SO₃ M, --OH,--OSO₃ M, --COOM, --PO(OM)₂, --NR₂ and --⊕NR₃ X⊖ (wherein M is H, Li,Na, K, or a hydrocarbon group having preferably 1 to 10 carbon atoms, Ris H, or an alkyl group having preferably 1 to 6 carbon atoms, and X isa halogen atom), ultrafine particles having a diameter of 5 to 250 nmare evenly dispersed, the height of convex portions formed on thesurface of the nonmagnetic intermediate layer is made relatively flat,therefore, the magnetic recording medium uniformly touches a magnetichead, and still life comes to increase. Further, there are formed on thesurface of the nonmagnetic intermediate layer minute convex and concaveportions the size and distribution of which are uniform, with the resultthat the contact surface between the magnetic recording medium and amagnetic head is decreased, and nevertheless the friction coefficientbecomes small. In addition, the skip of the magnetic head due to bigparticles can be prevented, therefore, clearance loss is made small, andthe decrease in output is lowered.

In the ferromagnetic thin film type magnetic recording medium of thepresent invention, when a binder resin in which the peak temperature ofthe loss tangent (tanδ) thereof is 10 to 90° C. is used, there can beformed a magnetic layer having a comparatively smooth surface.

As a result, the following excellent effects:

(1) the magnetic recording medium smoothly comes in contact with amagnetic head so that the still life is increased;

(2) the magnetic recording medium closely touches a magnetic head,therefore, clearance loss becomes small and the output is increased; and

(3) the running durability is increased.

The peak temperature of the loss tangent (tanδ) of the binder resin ofthe present invention is represented by the viscoelastic properties ofthe binder resin film, which are measured under a frequency of 110 Hz ata heat-up temperature of 2 degrees/minute. The measurement of the peaktemperature of the loss tangent is disclosed in Mechanical Properties ofPolymers, Chapter 7, written by Lawrence E. Nielsen and published byReinhold Publishing Corp., New York (1962).

In the present invention, the ultrafine particles for forming thenonmagnetic intermediate layer can be composed of various inorganicparticles or organic particles. Among them, inorganic particles such asTiO₂, Fe₂ O₃, CaCO₃, BaSO₄, SiO₂, Al₂ O₃, SiC and Cr₂ O₃ are preferablyused.

Inorganic particles having a Mohs' hardness of 6 or higher such as SiO₂,Al₂ O₃, SiC and Cr₂ O₃ are particularly preferred in terms of runningdurability.

Most preferred is SiO₂, particularly the so-called silica sol in whichamorphous silicic anhydride fine particles are dispersed in an organicsolvent.

The ultrafine particles have a diameter of 5 to 250 nm, preferably 10 to80 nm. When the size of the particles is too small, still life andrunning property are not significantly improved, whereas when their sizeis too large, clearance loss between a magnetic head and the magneticrecording medium is increased, thus leading to an insufficient output.

The diameter of the ultrafine particles refers to the maximum length ofthe particles.

The binder resin for forming the nonmagnetic intermediate layer of thepresent invention, which contains as a polar group at least one selectedfrom the group consisting of --SO₃ M, --OH, --OSO₃ M, --COOM, ##STR3##--NR₂ and --⊕NR₃ X⊖(wherein M is H, Li, Na, K or a hydrocarbon group, Ris H or an alkyl group, and X is a halogen atom), includes resins havingfilm-forming properties (e.g., cellulose resins such as nitro cellulose,thermoplastic resins and thermosetting resins such as acrylic resin andepoxy resin) and is preferably polyurethane, polyester or vinyl acetateresin, each having a number average molecular weight of about 10,000 to100,000, preferably about 30,000 to 80,000.

When the molecular weight is too small, the contact between theintermediate layer and the nonmagnetic support becomes poor, whereaswhen it is too large, it is difficult for the ultrafine particles to beuniformly dispersed. The concentration of the polar group is preferably1×10⁻² eq/g to 1×10⁻⁷ eq/g, more preferably 1×10⁻³ eq/g to 7×10⁻⁵ eq/g.When the concentration of the polar group is too high, the solubility ofthe binder resin in an organic solvent is decreased, whereas when it istoo low, it is difficult for ultrafine particles to be uniformlydispersed.

Examples of the binder resin in which the peak temperature of the losstangent (tanδ) is 10 to 90° C. include Stafix (polyester resin producedby Fuji Photo Film Co., Ltd.), Vylon #200 (polyester resin produced byToyobo Co., Ltd.), Vylonal #1400 (polyester resin produced by ToyoboCo., Ltd.), 400×110 A (polyvinyl chloride resin produced by Nippon ZeonCo., Ltd.), UR-8200 (polyurethane resin produced by Toyobo Co., Ltd.),and N-2301 (polyurethane resin produced by Nippon Polyurethane Co.,Ltd.). Among such binder resins, those which have as a polar group atleast one group selected from the group consisting of --SO₃ M, --OH,--OSO₃ M, --COOM, --PO(OM)₂, --NR₂ and --⊕NR₃ X⊖, (wherein M is H, Li,Na, K or a hydrocarbon group, R is H or an alkyl group, and X is ahalogen atom) can be preferably used in the present invention.

In the present invention, the peak temperature of the loss tangent(tanδ) of the binder resin is measured as follows. Namely, the binderresin is formed into a film of 25 μm thick, and the viscoelasticproperties of the film are measured under a frequency of 110 Hz at aheat-up temperature of 2 degrees/minute using a viscoelastic measuringdevice.

When the peak temperature of the loss tangent (tanδ) is less than 10°C., the surface of the intermediate layer, and thus the magnetic layer,is made uneven by heat when the magnetic layer is formed by evaporationor sputtering.

When the peak temperature of the loss tangent (tanδ) is more than 90°C., the adhesion ability between the nonmagnetic intermediate layer andthe nonmagnetic support is lowered, so that the magnetic layer is liableto be broken at the edge of the magnetic recording medium while it isrunning for a long period of time.

It is preferred that the binder resin has a number average molecularweight of 10,000 to 100,000, preferably 30,000 to 80,000.

The ultrafine particles are preferably contained in the nonmagneticintermediate layer at a rate of 1 to 100 particles/μm², more preferably3 to 70 particles/μm², most preferably 4 to 30 particles/μm². When therate is less than the lowermost value, μ value is increased and stilllife is lowered. When the rate is more than the uppermost value, noiseis increased.

In the nonmagnetic intermediate layer of the present invention, themixing ratio of the ultrafine particles to the binder resin is from 1/3to 1/80 by weight, preferably 1/5 to 1/60 by weight.

The ultrafine particles are dispersed in the binder resin to prepare acoating composition. The coating composition thus obtained is coated onthe nonmagnetic support, and then dried to form the nonmagneticintermediate layer.

The thickness of the nonmagnetic intermediate layer is preferably in therange of from 1 to 100 nm, more preferably 3 to 50 nm. If the thicknessis out of the range, the object of the present invention cannot beobtained.

In order to prepare the coating composition used to form the nonmagneticintermediate layer of the present invention, organic solvents can beadopted on arbitrary basis.

Examples of such organic solvents include ketones such as acetone,methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, andisophorone; alcohols such as methanol, ethanol, propanol, butanol,isobutyl alcohol, isopropyl alcohol, and methyl cyclohexanol; esterssuch as methyl acetate, ethyl acetate, butyl acetate, isobutyl acetate,isopropyl acetate, ethyl lactate, and glycol lactate monoethyl ether;ethers such as diethyl ether, tetrahydrofuran, glycol dimethyl ether,glycol monoethyl ether, and dioxane; aromatic hydrocarbons such asbenzene, toluene, xylene, cresol, chlorobenzene, and styrene;chlorinated hydrocarbons such as methylene chloride, ethylene chloride,carbon tetrachloride, chloroform, ethylene chlorohydrin, anddichlorobenzene; N,N-dimethyl formaldehyde; and hexane.

The nonmagnetic support used in the present invention can be made ofmaterials such as polyesters (e.g., polyethylene terephthalate,polyethylene-2,6-naphthalate), polyolefins (e.g., polyethylene,polypropylene), cellulose derivatives (e.g., cellulose triacetate),polycarbonate, polyimide, polyamide-imide, aluminum or an alloy thereof,copper, glass, or ceramics.

The ultrafine particles and the binder resin are well mixed and kneadedtogether using a roll mill, a sand grinder mill, a ball mill, a highspeed mixer, etc., to prepare the coating composition. The coatingcomposition thus prepared can be coated on the nonmagnetic support usinga bar coater, a coil bar coater, a micro gravure coater, a doctor bladecoater, a reverse roll coater, a curtain coater, an extrusion coater,etc., and then dried, thereby forming the nonmagnetic intermediatelayer. The temperature and air volume for drying can be optionallyselected.

A magnetic layer comprising a ferromagnetic thin film is provided on thenonmagnetic intermediate layer using a vacuum metallizing method such asa vacuum evaporation method, a sputtering method, an ion plating method,or a chemical vapor plating method. The vacuum metallizing method isadvantageous because a metal thin film can be formed rapidly, themanufacturing steps are simple, the anticorrosion is improved, and it isnot necessary to treat waste liquors.

The above mentioned ferromagnetic thin film can be made of materialssuch as iron, cobalt, nickel, ferromagnetic alloys (i.e., Fe-Co, Fe-Ni,Fe-N, Co-Ni, Fe-Si, Fe-Rh, Co-P, Co-B, Co-Si, Co-V, Co-Y, Co-La, Co-Ce,Co-Pr, Co-Sm, Co-Pt, Co-Mn, Fe-Co-Ni, Co-Ni-P, Co-Ni-B, Co-Ni-Ag,Co-Ni-Na, Co-Ni-Ce, Co-Ni-Zn, Co-Ni-Cu, Co-Ni-W, Co-Ni-Re, Co-Sm-Cu),Fe-N, or Fe-N-0. These materials are formed into a film in a vacuumchamber, or using a plating method.

The thickness of the ferromagnetic thin film can be 0.05 to 2 μm,preferably 0.1 to 0.4 μm.

In the present invention, when lubricating agents or rust preventivesare coated on the magnetic recording medium, the running properties canbe further increased. The lubricating agents or rust preventives may beadded to a backing layer. Examples of the lubricating agents includefatty acids, alkylsulfonic acids and metal salts thereof (e.g., stearicacid, oleic acid, lauric acid, octadecanoic acid sodium salt). Examplesof the rust preventives include 2-amino benzimidazole and 5-nitrobenzimidazole.

According to the present invention, a magnetic recording medium having aferromagnetic thin film as a magnetic layer and having great output aswell as excellent running durability can be obtained by the use of anonmagnetic intermediate layer comprising a binder resin having a polargroup or a binder resin having the peak temperature of the loss tangent(tanδ) of 10 to 90° C., and ultrafine particles having a diameter of 5to 250 nm.

The present invention is described in greater detail with reference tothe following examples, although it is not limited thereto.

EXAMPLE 1

A coating composition comprising components A, B and C, which are shownbelow, was coated on a nonmagnetic support of a polyethyleneterephthalate film which was 10 μm thick and 100 mm wide, and thereafterwas forcibly dried with heated air of 100° C. to obtain a nonmagneticintermediate layer having a dry thickness of 8 nm.

Component A

A solution having a solid content of 10% consisting of polyester resin(neopentyl glycol-isophthalic acids) having a molecular weight of 45,000as a binder resin and --SO₃ Na of 3.5×10⁻³ eq/g as a polar group andcyclohexanone/MEK (weight ratio : 6/4)

12 parts by weight

Component B

A solution having a solid content of 5% consisting of silica colloid sol(SiO₂ particle size:20 nm) as ultrafine particles and cyclohexanone/MEK(weight ratio:6/4)

2 parts by weight

Component C

Cyclohexanone/MEK (weight ratio:6/4)

786 parts by weight

A magnetic thin film of Co-Ni (Ni:20 atom%) was formed on the abovenonmagnetic intermediate layer by an oblique incident evaporation methodusing a winding type vacuum vapor deposition device shown in thedrawing, thereby preparing an original magnetic tape web. The incidentangle 17 of a vapor flow being regulated by mask 13 was 35 degrees. Theangle means the angle formed by a normal put up on cooling can 9 and avapor flow 16 of Co-Ni. The vapor deposition was carried out whileoxygen gas was introduced from gas inlets 14 and 15.

The carrying speed of nonmagnetic support 6 was 20 m/min, and thethickness of the magnetic layer was made to be 2000 Å. Aperfluoropolyether-based lubricating agent was coated on the magneticlayer so that the thickness thereof could be 80 Å.

The tape web which was wound by the winding roll 8 was slit to a 8 mmwidth, thereby obtaining sample No. 1 corresponding to a ferromagneticthin film type magnetic recording medium for a 8 mm video tape.

In addition, the polyester resin used for forming the binder resin wasdissolved in a solvent of methyl ethyl ketone/toluene (weight ratio:9/1)to prepare a 20% solution of the resin. The solution thus prepared wastreated using a casting method to obtain a resin film having a drythickness of 25 μm.

The dynamic modulus of such binder resin film was measured at a heat-uptemperature of 2 degrees/minute under a vibrating frequency of 110 Hzusing a dynamic viscoelastic measuring device, and the peak temperatureof the loss tangent (tanδ) of the binder resin was found to be 65° C.

EXAMPLE 2

The same procedure as in Example 1 was repeated to prepare Sample No. 2of a ferromagnetic thin film type magnetic recording medium except thatthe polyester resin in component A was replaced with polyvinyl chlorideresin having a molecular weight of 24,000 and a --SO₃ Na content of5.5×10⁻⁴ eq/g.

EXAMPLE 3

The same procedure as in Example 1 was repeated to prepare Sample No. 3of a ferromagnetic thin film type magnetic recording medium except thatthe polyester resin in component A was replaced with polyurethane resinhaving a molecular weight of 25,000 and a --COOH content of 3.6×10⁻⁵eq/g.

COMPARATIVE EXAMPLE 1

The same procedure as in Example 1 was repeated to prepare Sample No.C-1 of a ferromagnetic thin film type magnetic recording medium exceptthat the polyester resin in component A was replaced with a vinylchloride/vinyl acetate copolymer (78/22 weight ratio; polymerizationdegree: 420) having no polar groups.

COMPARATIVE EXAMPLE 2

The same procedure as in Example 1 was repeated to prepare Sample No.C-2 of a ferromagnetic thin film type magnetic recording medium exceptthat the polyester resin in component A was replaced with polyesterresin (molecular weight: 45,000) having no polar groups.

EXAMPLE 4

The same procedure as in Example 1 was repeated to prepare Sample Nos. 4to 10 of a ferromagnetic thin film type magnetic recording medium exceptthat the silica colloid sol in component B was replaced with thefollowing ultrafine particles each having a diameter of not larger than60 nm.

    ______________________________________                                        Sample No. 4           Al.sub.2 O.sub.3                                       Sample No. 5           SiC                                                    Sample No. 6           Cr.sub.2 O.sub.3                                       Sample No. 7           TiO.sub.2                                              Sample No. 8           Fe.sub.2 O.sub.3                                       Sample No. 9           CaCO.sub.3                                             Sample No. 10          BaSO.sub.4                                             ______________________________________                                    

EXAMPLE 5

The same procedure as in Example 1 was repeated to prepare Sample Nos.11 to 16 of a ferromagnetic thin film type magnetic recording mediumexcept that the concentration of --SO₃ Na in component A was replacedwith the following:

    ______________________________________                                        Sample No.       Concentration of SO.sub.3 Na                                 ______________________________________                                        No. 11           1 × 10.sup.-1 eq/g                                     No. 12           1 × 10.sup.-3 eq/g                                     No. 13           1 × 10.sup.-4 eq/g                                     No. 14           1 × 10.sup.-6 eq/g                                     No. 15           1 × 10.sup.-7 eq/g                                     No. 16           1 × 10.sup.-8 eq/g                                     ______________________________________                                    

The polyester resin used in Sample No. 11 was insufficient to be solvedin the solvent (cyclohexanone/MEK). The dispersibility of the silicacolloid sol in component B was not good so that an aggregate was formed.In the polyester resin used in Sample No. 16, the dispersion of thesilica colloid sol in component B was insufficient, thus forming anaggregate.

EXAMPLE 6

The same procedure as in Example 1 was repeated to prepare Samples Nos.17 to 22 of a ferromagnetic thin film type magnetic recording mediumexcept that the disposition density of ultrafine particles in componentB was replaced with the following:

    ______________________________________                                                        Disposition density of                                        Sample No.      ultrafine particles                                           ______________________________________                                        No. 17           0 particles/μm.sup.2                                                      (No component is added)                                       No. 18           1 particle/μm.sup.2                                       No. 19           5 particles/μm.sup.2                                      No. 20          10 particles/μm.sup.2                                      No. 21          15 particles/μm.sup.2                                      No. 22          30 particles/μm.sup.2                                      ______________________________________                                    

Regarding Sample Nos. 1 to 22, and Sample Nos. C-1 to C-2 thus obtained,their still lives, outputs at 7 MHz, drops in output for 120 minutes for100 passes, and uniform dispersibilities of ultrafine particles innonmagnetic intermediate layers were measured in the following manner.Table 1 shows the results.

All the measuring devices used were remodeled ones of "Fujix M-6"produced by Fuji Photo Film Co., Ltd.

(1) Still life

It was measured at 23° C. and 20% RH in a state in which 20 g in weightwas applied to the tape. The time (minutes) for which the first outputis decreased to 3 dB was measured.

(2) Output at 7 MHz

It was shown in terms of a relative value with respect to Sample No. 1which was assigned to a standard (0 dB) when recording was done at 7MHz. The measurement was conducted at 23° C. and 50% RH.

(3) Drop in output for 120 minutes for 100 passes

The decreased amount of RF output from the initial value was measuredwhen color bar signals were recorded. The measurement was conducted at23° C. and 50% RH.

(4) Uniform dispersibilitv

It was evaluated using an FE-SEM of 50,000 magnifications.

                                      TABLE 1                                     __________________________________________________________________________    Sample                                                                            Uniform          Output at                                                                            Drop in output for                                No. dispersibility                                                                          Still life                                                                           7 MHZ (dB)                                                                           120 min. for 100 passes                           __________________________________________________________________________     1  good      >60 minutes                                                                          0 (Standard)                                                                         -0.3 dB                                            2  good      >60 minutes                                                                          +0.1   -0.2 dB                                            3  good      >100 minutes                                                                         +0.1   -0.2 dB                                           C-1 much aggregation                                                                        5 minutes                                                                            -3.2   -2.8 dB                                           C-2 much aggregation                                                                        13 minutes                                                                           -2.5   -3.0 dB                                            4  good      >60 minutes                                                                          +0.1   -0.2 dB                                            5  good      >60 minutes                                                                          +0.1   -0.1 dB                                            6  good      >60 minutes                                                                          0      -0.2 dB                                            7  3% aggregation                                                                          >60 minutes                                                                          -0.1   -0.3 dB                                            8  4% aggregation                                                                          >60 minutes                                                                          -0.2   -0.3 dB                                            9  4% aggregation                                                                          >60 minutes                                                                          -0.2   -0.2 dB                                           10  7% aggregation                                                                          >60 minutes                                                                          -0.3   -0.3 dB                                           11  not evenly dispersed                                                                    <1 minute.sup.                                                                       -3.8   -3.1 dB                                           12  good      >60 minutes                                                                          0      -0.2 dB                                           13  good      >60 minutes                                                                          0      - 0.2 dB                                          14  good      >60 minutes                                                                          +0.1   -0.1 dB                                           15  good      >60 minutes                                                                          +0.1   -0.1 dB                                           16  much aggregation                                                                        8 minutes                                                                            -3.0   -3.5 dB                                           17  good      >60 minutes                                                                          0      -0.1 dB                                           18  good      >60 minutes                                                                          0      -0.1 dB                                           19  good      >60 minutes                                                                          +0.1   -0.1 dB                                           20  good      >60 minutes                                                                          -0.1   -0.1 dB                                           21  good      >60 minutes                                                                          +0.2   -0.2 dB                                           22  good      >60 minutes                                                                          -2.5   -0.2 dB                                           __________________________________________________________________________

EXAMPLE 7

The same procedure as in Example 1 was repeated to prepare Sample No. 23of a ferromagnetic thin film type magnetic recording medium for an 8 mmvideo tape except that the polyester resin in component A was replacedwith a polyester resin in which the ratio of neopentylglycol/isophthalic acid was varied and the peak temperature of the losstangent (tanδ) thereof was 18° C.

EXAMPLE 8

The same procedure as in Example 1 was repeated to prepare Sample No. 24of a ferromagnetic thin film type magnetic recording medium for an 8 mmvideo tape except that the polyester resin in component A was replacedwith a polyester resin in which the ratio of neopentylglycol/isophthalic acid was varied and the peak temperature of the losstangent (tanδ) thereof was 85° C.

COMPARATIVE EXAMPLE 3

The same procedure as in Example 1 was repeated to prepare Sample No.C-3 of a ferromagnetic thin film type magnetic recording medium for an 8mm video tape except that the polyester resin in component A wasreplaced with a polyester resin in which a ratio of neopentylglycol/isophthalic acid was varied and the peak temperature of the losstangent (tanδ) thereof was 4° C.

COMPARATIVE EXAMPLE 4

The same procedure as in Example 1 was repeated to prepare Sample No.C-4 of a ferromagnetic thin film type magnetic recording medium for an 8mm video tape except that the polyester resin in component A wasreplaced with a polyester resin in which a ratio of neopentylglycol/isophthalic acid was varied and the peak temperature of the losstangent (tanδ) thereof was 10° C.

COMPARATIVE EXAMPLE 5

The same procedure as in Example 1 was repeated to prepare Sample No.C-5 of a ferromagnetic thin film type magnetic recording medium for an 8mm video tape except that a silica colloid sol having a SiO₂ particlesize of 3 nm was used as component B.

COMPARATIVE EXAMPLE 6

The same procedure as in Example 1 was repeated to prepare Sample No.C-6 of a ferromagnetic thin film type magnetic recording medium for an 8mm video tape except that a silica colloid sol having a SiO₂ particlesize of 300 nm was used as component B.

The results of Sample Nos. 23, 24, and C-3 to C-6 are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________    Sample                                                                            Uniform                                                                              Still life                                                                         Output at                                                                           Drop in output for                                                                       Friction                                     No. dispersibility                                                                       (minute)                                                                           7 MHz (dB)                                                                          120 min. for 100 passes                                                                  coefficient                                  __________________________________________________________________________     23 Good   >60  0     -0.3 dB    0.23                                         24  Good   >60  0     -0.2 dB    0.23                                         C-3 Good   >60  -2.4  -1.4 dB    0.23                                         C-4 Good    45  -0.2  -2.6 dB    0.21                                         C-5 Aggregation                                                                           13  -0.2  -2.3 dB    0.30                                         C-6 Good   >60  -3.1  -0.2 dB    0.20                                         __________________________________________________________________________

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A ferromagnetic thin film type magnetic recordingmedium comprising a nonmagnetic intermediate layer having a thickness inthe range of from 1 to 100 nm formed between a nonmagnetic support and amagnetic layer, aid intermediate layer containing a binder resin havinga number average molecular weight of about 10,000 to 100,000 andultrafine particles at a rate of 1 to 100 particles/μm², wherein saidultrafine particles have a diameter of from 5 to 250 nm and (a) saidbinder resin has as a polar group at least one group in a concentrationof from 1×10⁻² eq/g to 1×10⁻⁷ eq/g selected from the group consisting of--SO₃ M,--OH, --OSO₃ M, --COOM, ##STR4## --NR₂ and --⊕NR₃ X⊖wherein M isH, li, na, K or a hydrocarbon group, R is H or an alkyl group, and X isa halogen atom, or (b) the peak temperature of the loss tangent (tanδ)of said binder resin is from 10 to 90°) C.
 2. A ferromagnetic recordingmedium as in claim 1, wherein the peak temperature of the loss tangent(tanδ) of said binder resin is from 10 to 90° C. and said binder resincontains said polar group.
 3. A ferromagnetic recording medium as inclaim 1, wherein the peak temperature of the loss tangent (tanδ) of saidbinder resin is from 15 to 85° C.
 4. A ferromagnetic recording medium asin claim 1, wherein the peak temperature of the loss tangent (tanδ) ofsaid binder resin is from 18 to 80° C.
 5. A ferromagnetic recordingmedium as in claim 1, wherein the diameter of said ultrafine particlesis 10 to 80 nm.
 6. A ferromagnetic recording medium as in claim 1,wherein the binder resin has a number average molecular weight of about30,000 to 80,000.
 7. A ferromagnetic recording medium as in claim 1,wherein the concentration of the polar group is 1×10⁻³ : eq/g to 7×10⁻⁵eq/g.
 8. A ferromagnetic recording medium as in claim 1, wherein saidultrafine particles are contained in the nonmagnetic intermediate layerat a rate of 4 to 30 particles/μm².
 9. A ferromagnetic recording mediumas in claim 1, wherein the mixing ratio of said ultrafine particles tosaid binder resin is from 1/3 to 1/80 by weight.
 10. A ferromagneticrecording medium as in claim 1, wherein the mixing ratio of saidultrafine particles to said binder resin is from 1/5 to 1/60 by weight.11. A ferromagnetic recording medium as in claim 1, wherein thethickness of the nonmagnetic intermediate layer is in the range of from3 to 50 nm.
 12. A ferromagnetic recording medium as in claim 1, whereinthe nonmagnetic intermediate layer further contains an organic solvent.